1. Field of the Invention
This invention relates to mosaic needle printing heads and more particularly to needle printing systems utilizing printing needles activated by linearly moving magnet armatures.
2. Prior Art
A known type of printing mechanism utilizes a printing head having a mosaic of blunt ended needles selectively projectable from a printing head face. One method of moving the needles to bring them into projected printing positions is to attach the end of the needle opposite the printing end to the armature of a plunger type magnet system such as a miniature magnetic solenoid.
In the manufacture of such mosaic needle printing devices, the interconnection between the individual printing needles and the armatures of the electromagnet systems creates a number of relatively unique and critical problems. Many of these problems can be identified as being a function of the relatively small size of the printing needles used in such printing mechanisms. Needles having a diameter less than 0.4 mm are common. Thus dimensional restrictions create a number of problems which would not be encountered in larger plunger type electromagnetic systems.
As a further complicating factor, in order to obtain a sufficiently long service life the printing needles themselves have to be both wear and fracture resistent. Those materials which exhibit the desired wear and fracture resistent properties have been found, through experience, to be relatively incompatible with the optimum materials for construction of the electromagnet armatures. A particular area of incompatibility lies in the joining together of the needle and the armature. Again, due to the size of the overall system, it is very important to choose the material for the electromagnet armature with a high regard for its ferromagnetic property as opposed to choosing an armature material having the best connection compatibility with the needle.
Yet another series of problems arises from the fact that when utilizing plunger type armature electromagnetic systems in mosaic needle printing devices, the connection of the printing needle into the receiving bore hole in the armature requires that the bore have a small diameter approximating the diameter of the needle while having a relatively long depth. These very small diameter but long bores have to be centrally placed in the armature. Production of such bore holes within required tolerance limits is very expensive.
Attempting to make the connection between the armature and the needle by heat connection systems such as soldering or welding adds additional complexity. The heat treatment required by such connection systems often creates structural damage to the materials being used, both to the armature and to the needle, which structural damage is aggravated by the very small dimensions used.
In any type of connection system, close attention has to be paid to insure that the printing needle and the armature are substantially completely axially aligned. Failure to obtain proper alignment will introduce a tilting of the armature in the magnet which will thereafter result in an improper catching of the armature during movement.
Nor can these difficulties be overcome through the standard expedients press fit or interference fit of the needle end into the armature. The material of the armature will not have, due to the very small dimension of the bore hole, a sufficient elasticity to provide a desired tight grip on the needle.
Because of some of the above difficulties, it has been suggested to provide printing needles which are received in blind ended bores in the armature and which are held along the cylindrical side wall of the bores by elastically deformable means. One line of reasoning for this type of construction proceeds from the fact that the forces which effect the fastening between the plunger type armature and the printing needle are extremely diverse. Such forces are applied from many differing directions during printing however their strength varies according to direction. Thus forces securing the needle in the armature can vary according to different directions while still providing a sufficiently safe functional fastening. The greatest force occurs axially and such blind hole bores positively intercept the force at the base of the blind hole whereas forces opposite to the printing force are relatively lesser and can be accounted for by the frictional grip of the needle along the length of the cylindrical wall.